The present invention relates to an illumination unit comprising at least one light source device and a planar light waveguide for illuminating a controllable reflective spatial light modulator, where the light waveguide comprises a light conducting core and a cladding, where the light modulator comprises a pixel matrix, where the light source device is arranged on the side of the light waveguide and where the light emanating from at least one light source propagates areally though the light waveguide. The spatial light modulator is designed to serve as a display panel of a direct-view display.
Illumination units can come as backlights or frontlights (also referred to as transmitted-light and reflected-light illumination devices, respectively) and generally serve to illuminate a transmissive or reflective controllable spatial light modulator (SLM) of a direct-view display. The light can be coherent or incoherent. Display devices which are operated with incoherent light are preferably used as 2D displays for autostereoscopic 3D presentations. Coherent light is required for example in holographic display devices.
The field of application of the present invention includes direct-view displays for the three-dimensional presentation of autostereoscopic and holographic images.
In a commercially available flat TV display for the presentation of two-dimensional images or videos, it is necessary to realise a bright and homogeneous illumination of the entire surface at high resolution. The SLM which serves as display panel is required to emit the light in a large angular range. Many physical forms of such displays are known in the prior art.
Most of them have a planar optical light waveguide (LWG). The planar LWG generally comprises at least one light conducting core and a cladding, both of which differing in the refractive index. The injected light propagates through the planar LWG in the form of pencils of rays or wave fields under the conditions of total internal reflection (TIR) and is coupled out to illuminate the display panel. Alternatively, the light is conducted without being reflected and coupled out through the cladding as evanescent wave fields of different modes m.
A number of issues need to be considered in a display device with backlight or frontlight and planar light waveguide to be able to realise an optimally designed illumination device. First, this relates to the physical form of a planar LWG itself, including the mechanisms for injecting and coupling out the light. Secondly, this relates to the physical form of the light source device including the light sources which supply the light. Further, it must be taken into consideration whether the display device is of a transmissive or reflective type.
In contrast to a flat TV display, an illumination unit in a autostereoscopic or holographic display device for the three-dimensional presentation of information must satisfy a number or further or different requirements. The information to be presented is written to the SLM of the display device. The light which is emitted by the light sources is modulated with the information that is written to the SLM, where the SLM typically at the same time serves as screen or display panel. It is therefore necessary to strictly ensure parallel incidence of the pencils of rays onto the SLM and to achieve a high refresh rate of the SLM.
SLM with very high refresh rates of for example 240 fps (frames per second) are required to be able to realise a three-dimensional presentation of information. The SLM panels which are used for light modulation in the display devices are often liquid crystal (LC) type SLMs, such as reflective SLMs of the LCoS type, which are currently achieving refresh rates of more than 400 fps in high definition (HD). A refresh rate of 1085 fps has already been realised with an LCoS array with 256×256 pixels, which was run at 15 V.
In contrast to transmissive SLMs, reflective SLMs often comprise a higher fill factor, thus allowing for an advanced suppression of undesired diffraction orders of coherent light if used in holographic display devices.
In LC displays, halving the thickness of the LC layer means to increase the refresh rate fourfold. Further, doubling the voltage also results in a fourfold increase in the maximum achievable refresh rate. This is because both the thickness of the LC layer and the voltage which is applied to the LC layer are squared in the expression used to calculate the maximum achievable refresh rate. With a non-transparent circuit carrier (backplane), increasing the voltage, frequency and current can be realised easily, because here the entire surface area is available for conductors, transistors and capacitors.
In addition to the necessary high refresh rate, great demands are made on the collimated emission of the light by the LWG. To achieve a high quality of the 3D presentation of the information, a defined collimation of the wave fronts that are coupled out is necessary in addition to a homogeneous illumination of the entire surface of the SLM. This is of particular importance for holographic presentations in the form of a reconstruction that is to be generated. The holographic information, which can for example be an object that is composed of object points of a three-dimensional scene, is encoded in the form of amplitude and phase values in the pixels of the SLM. Each encoded object point is represented by a wave front that is emitted by the SLM.
The angular range of a wave front that is emitted by the illumination unit is referred to as the ‘angular spectrum of plane waves’. It has been found in practice that an angular spectrum of plane waves where the plane wave fronts comprise mutual deviations in the emission angle of more than 1/60° in the coherent direction will result in a blurred reconstructed object point. This blur can be perceived by the eye under optimum conditions. The emission angle of the spectrum of plane waves of a holographic display should therefore lie at least in the range of between 1/70° and 1/40° in the coherent direction. In the incoherent direction, it should be wide enough to illuminate at least the eye pupil.
Consequently, the collimated wave fronts which illuminate the SLM must a priori have a defined emission angle in relation to each other in order to circumvent the negative illumination-induced effects on the reconstruction to be generated. In autostereoscopic 3D presentations, the collimation of the pencils of rays enhances the image quality of the display device. The angular spectrum of plane waves should here be chosen such that the eye pupil of the other eye is not illuminated.
Collimated emission of coherent light can for example be achieved by using volume gratings which are arranged on or in the planar LWG. They represent a stack of transparent layers and can be described as modulated distributions of refractive indices in the X and Y direction; there are transmissive and reflective volume gratings. A 3D volume grating is generated by interference of two or more coherent or at least partly coherent waves. The structure of the volume grating is determined by parameters such as the wavelength in the material and the local angles between interfering wave fronts of the light used for recording. A volume grating is generally made such that a defined portion of energy can be coupled out in a specified angular range. Bragg's diffraction conditions apply to those gratings during reconstruction.
However, in order to be able to realise a limitation of the angular spectrum of plane waves that is coupled out of smaller than 1/20° with an illumination unit with planar light waveguide and volume grating as proposed in this invention, the volume grating is required to have a thickness of about 500 μm. Now, if the angular resolving power limit of the human eye of 1/60° is taken into consideration, the volume grating must have a layer thickness of e.g. 1 mm. The angular selectivity depends on the actual geometry of the reconstruction.
This fact is derived from Kogelnik's ‘coupled wave theory’. However, this theory is only derived for volume gratings which are reconstructed in the first Bragg's diffraction order, i.e. it only applies to those.
Recording the holographic grating which works for example in a total internal reflection geometry, is technologically complex according to this theory, because very large angles had to be realised between the interfering wave fronts. Large prisms and a liquid index matching material (oil) are required to achieve large deflection angles. Further, this design will cause great layer thicknesses, a narrow angular selectivity of the volume grating and small grating periods which come close to the resolution limit of the available materials.
It is therefore an object to make the manufacture of the volume gratings which are required in the illumination unit more inexpensive.
There are further problems that need being taken into consideration in conjunction with the illumination unit with a volume grating.
If the light which propagates for example by way of total internal reflection is well collimated, then a wide angular selectivity is advantageous for easy adjustment. This can be achieved by reflective volume gratings, because they comprise a wider angular selectivity than transmissive volume gratings.
The thicker the volume gratings, the more reduced is the angular selectivity of the diffraction efficiency η(θin). This means that a high diffraction efficiency near 1 is only available at a small angle. This can be taken advantage of to only couple a narrow angular range out of a light conducting layer.
If the collimation of the light which propagates for example by way of total internal reflection is too wide, then it is advantageous to realise a sufficiently narrow angular selectivity to get a narrow angular spectrum of plane waves. This is achieved with thick transmissive volume gratings.
An adaptation to the light that is actually to be coupled out can be achieved by choosing the parameters of the volume gratings accordingly.
Further, it must be noted that the grating period becomes the smaller the larger the emission angle of the light which is coupled out. This may bring about a resolution problem for the grating material that is used in the volume grating. Moreover, the resolving power limit of the human eye, which is about 1/60°, must be taken into account when producing the volume grating. If this limit is taken into account, the illumination unit e.g. in a holographic display must realise an angular spectrum of plane waves that ranges between 1/20° and 1/60° in order to illuminate the SLM with well collimated light.
A typical eye separation measures 65 mm. Given a distance to the display panel of 1 m, this corresponds to an angle of 3.72°. At an observer distance of 1 m, this is the geometric limit of the angular range of the plane waves emitted by the light waveguide in the incoherent direction from which cross-talking to the other eye occurs.
Widening of the diffraction order does not only occur in the coherent direction, but also in the incoherent direction. When taking this fact into consideration, the emission angle in the incoherent direction shall be chosen smaller than would be necessary according to the geometric-optical calculation.
Planar light waveguides are preferably used in illumination units of flat displays so to realise the flatness of those display devices. They are designed with the help of additional optical components such that the light is preferably emitted by the display in a large angular range in order to enlarge the viewing space in front of the display.
Document U.S. Pat. No. 6,648,485 B1 discloses a wedge-shaped light waveguide, i.e. one which is not coplanar, in which the light propagates by way of multiple reflections and which is used for homogeneous illumination of a flat display. In order to control the angle-dependent distribution of the light which is injected into the light waveguide, the entry surface of the wedge is for example fitted with a scattering surface profile. Further, the wedge is dimensioned such that the light leaves the frustrated total internal reflection (FTIR) condition during its propagation through the light waveguide.
However, to ensure the angular selectivity that an illumination unit of a holographic display device is required to have the wedge angle had to be much smaller than 1°. This is not realistic with a light waveguide according to that document.
Document JP 2007234385 A discloses a backlight with wedge-shaped light waveguide for a flat display, where the backlight comprises coloured LED light sources. Their light can be injected into the wedge in a divergent, convergent or parallel manner by reflectors which are designed in the form of paraboloid mirrors. The object is a homogeneous illumination of the entire surface of the flat display. Referring to FIG. 14 of that document, the exit angles of the light which leaves the light waveguide at an oblique angle are affected by subsequent optical components, e.g. a prism plate, such that the propagation angle of the light is much larger than 1/60°.
In document WO 2004/109380 A1, light emitted by light sources is injected into the widest face of the wedge-shaped waveguide of a flat display through a cylindrical mirror. It propagates through the waveguide by way of multiple reflections. The emitted light is distributed homogeneously across the waveguide by a prism foil, where the emission angle is not smaller than 15°.
Flat displays with light waveguides which are known in the prior art, including those described in the above-mentioned documents, are not suited due to their emission characteristics to satisfy the great demands which are made on an illumination unit of a fast switching display device. They do not offer the possibility to generate a near-flawless reconstruction of an object in a holographic direct-view display device.